Method for fabricating a semiconductor device

ABSTRACT

Concentration of metal element which promotes crystallization of silicon and which exists within a crystalline silicon film obtained by utilizing the metal element is reduced. A first heat treatment for crystallization is performed after introducing nickel to an amorphous silicon film  103 . Then, laser light is irradiated to diffuse nickel element which is concentrated locally. After that, another heat treatment is performed within an oxidizing atmosphere at a temperature higher than that of the previous heat treatment. At this time, HCl or the like is added to the atmosphere. A thermal oxide film  106  is formed in this step. At this time, gettering of the nickel element into the thermal oxide film  106  takes place. Then, the thermal oxide film  106  is removed. Thereby, a crystalline silicon film  107  having low concentration of the metal element and a high crystallinity can be obtained.

FIELD OF THE INVENTION

The present invention relates to a semiconductor device typified by athin film transistor and to a fabrication method thereof. Specifically,the present invention relates to a semiconductor device using acrystalline silicon thin film formed on a glass substrate or a quartzsubstrate and to a fabrication method thereof.

DESCRIPTION OF RELATED ART

Hitherto, there has been known a thin film transistor using a siliconfilm, i.e. a technology for forming the thin film transistor by usingthe silicon film formed on a glass substrate or quartz substrate.

The glass substrate or quartz substrate is used because the thin filmtransistor is used for an active matrix type liquid crystal display.While a thin film transistor has been formed by using an amorphoussilicon film in the past, it is being tried to fabricate the thin filmtransistor by utilizing a silicon film having a crystallinity (referredto as “crystalline silicon film” hereinbelow) in order to enhance itsperformance.

The thin film transistor using the crystalline silicon film can operateat high speed by more than two digits as compared to one using anamorphous silicon film. Therefore, while peripheral driver circuits ofan active matrix liquid crystal display have been composed of externalIC circuits, they may be built on the glass substrate or quartzsubstrate similarly to the active matrix circuit.

Such structure is very advantageous in miniaturizing the whole apparatusand in simplifying the fabrication process, thus leading to reduction ofthe fabrication cost.

In general, a crystalline silicon film has been obtained by forming anamorphous silicon film by means of plasma CVD or low pressure thermalCVD and then by crystallizing it by performing a heat treatment or byirradiating laser light.

However, it has been difficult to obtain a required crystallinity acrossthe wide area through the heat treatment because it may causenonuniformity in the crystallization.

Further, although it is possible to obtain the high crystallinity partlyby irradiating laser light, it is difficult to obtain a good annealingeffect across the wide area. In particular, the irradiation of the laserlight is apt to become unstable under the conditions needed forobtaining the good crystallinity.

Meanwhile, a technology described in Japanese Patent Laid-Open No. Hei.6-232059 has been known. This technology obtains a crystalline siliconfilm through a heat treatment at a lower temperature than that of theprior art by introducing a metal element (e.g. nickel) which promotesthe crystallization of silicon to the amorphous silicon film.

This technology allows high crystallinity to be obtained uniformlyacross a wide area as compared to the prior art crystallization methodby way of only heating or crystallization of an amorphous silicon filmonly by means of irradiation of laser light.

However, it is difficult to obtain a crystalline silicon film havinghigh crystallinity and homogeneity across a wide area which is requiredfor an active matrix type liquid crystal display.

Further, because the metal element is contained within the film and anamount thereof to be introduced has to be controlled very carefully,there is a problem in its reproducibility and stability (electricalstability of a device obtained).

Still more, there is a problem that an elapsed change of thecharacteristics of a semiconductor device to be obtained is large or anOFF value, in case of a thin film transistor, is large, due to theinfluence of the remaining metal element.

That is, although the metal element which promotes the crystallizationof silicon plays the useful role in obtaining the crystalline siliconfilm, its existence becomes a negative factor which causes variousproblems after obtaining the crystalline silicon film once.

SUMMARY OF THE INVENTION

It is an object of the invention disclosed in the present specificationto provide a semiconductor device having excellent characteristics byusing a crystalline silicon film having a high crystallinity.

It is an object of the invention disclosed in the present specificationto provide a technology for reducing concentration of a metal elementwithin a crystalline silicon film obtained by utilizing the metalelement which promotes crystallization of silicon.

It is another object of the present invention to provide a technologywhich can enhance characteristics and reliability of the semiconductordevice thus obtained.

One of the inventions disclosed in the present specification ischaracterized in that it comprises steps of intentionally introducing ametal element which promotes crystallization of silicon to an amorphoussilicon film and crystallizing the amorphous silicon film by a firstheat treatment to obtain a crystalline silicon film; irradiating laserlight or intense light to the crystalline silicon film; removing orreducing the metal element existing within the crystalline silicon filmby performing a second heat treatment within an oxidizing atmospherecontaining a halogen element; removing a thermal oxide film formed inthe previous step; and forming another thermal oxide film on the surfaceof the region from which the thermal oxide film has been removed byperforming another thermal oxidation.

An arrangement of another invention is characterized in that itcomprises steps of intentionally introducing a metal element whichpromotes crystallization of silicon to an amorphous silicon film andcrystallizing the amorphous silicon film by a first heat treatment toobtain a crystalline silicon film; irradiating laser light or intenselight to the crystalline silicon film to diffuse the metal element,existing within the crystalline silicon film, in the crystalline siliconfilm; performing a second heat treatment within an oxidizing atmospherecontaining a halogen element to cause the metal element existing withinthe crystalline silicon film to be gettered to a thermal oxide film tobe formed; removing the thermal oxide film formed in the previous step;and forming another thermal oxide film on the surface of the region fromwhich the thermal oxide film has been removed by performing anotherthermal oxidation.

An arrangement of another invention is characterized in that itcomprises steps of intentionally introducing a metal element whichpromotes crystallization of silicon to an amorphous silicon film andcrystallizing the amorphous silicon film by a first heat treatment toobtain a crystalline silicon film; forming an active layer of thesemiconductor device by patterning the crystalline silicon film;irradiating laser light or intense light to the active layer; performinga second heat treatment within an oxidizing atmosphere containing ahalogen element to remove or reduce the metal element existing withinthe active layer; removing a thermal oxide film formed in the previousstep; and forming another thermal oxide film on the surface of theactive layer by performing another thermal oxidation.

An arrangement of another invention is characterized in that itcomprises steps of intentionally and selectively introducing a metalelement which promotes crystallization of silicon to an amorphoussilicon film; performing a first heat treatment to the amorphous siliconfilm to grow crystal in a direction parallel to the film from a regionof the amorphous silicon film into which the metal element has beenintentionally and selectively introduced; irradiating laser light orintense light to diffuse the metal element existing within the regionwhere the crystal has grown; performing a second heat treatment withinan oxidizing atmosphere containing a halogen element to cause the metalelement existing within the region where the crystal has grown to begettered to a thermal oxide film to be formed; removing the thermaloxide film formed in the previous step; and forming another thermaloxide film on the surface of the region from which the thermal oxidefilm has been removed by performing another thermal oxidation.

An arrangement of another invention is characterized in that itcomprises steps of intentionally introducing a metal element whichpromotes crystallization of silicon to an amorphous silicon film andcrystallizing the amorphous silicon film by a first heat treatment toobtain a crystalline silicon film; forming an active layer of thesemiconductor device by patterning the crystalline silicon film;irradiating laser light or intense light to the active layer; performinga second heat treatment within an oxidizing atmosphere containing ahalogen element to remove or reduce the metal element existing withinthe active layer; removing a thermal oxide film formed in the previousstep; and forming another thermal oxide film on the surface of theactive layer by performing another thermal oxidation, wherein the activelayer has a tapered shape in which an angle formed between a side faceand an underlying face is 20° to 50°.

In the invention disclosed in the present specification, one or aplurality elements selected from Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Cuand Au may be used as the metal element which promotes thecrystallization of silicon.

Further, as the oxidizing atmosphere containing the halogen element, anatmosphere to which one or a plurality of gases selected from HCl, HF,HBr, Cl₂, F₂ or Br₂ is added into an O₂ atmosphere or an atmospherecontaining O₂ may be used.

The concentration of impurity in the present specification is defined asthe minimum value of measured values measured by the SIMS (secondary ionmass spectrometry).

According to a preferred mode for carrying out the invention disclosedin the present specification, an amorphous silicon film is formed atfirst. Then, the amorphous silicon film is crystallized by an action ofmetal element typified by nickel which promotes crystallization ofsilicon to obtain a crystalline silicon film. The crystallization iscarried out by heat treatment.

This heat treatment is performed within a range of 550° C. to 750° C. Itis preferable to perform the heat treatment at a temperature above 620°C.

The metal element is contained in the crystalline silicon film in thestate in which the film has been crystallized by the above-mentionedheat treatment.

Here, laser light is irradiated to promote the crystallization of thecrystalline silicon film obtained and to diffuse (disperse) the nickelelement existing within the film further within the film at the sametime.

In the state in which the film is crystallized by the first heattreatment, the nickel element exists as certain blocks. Then, the nickelelement may be diffused by a certain degree so that it may be readilygettered later by irradiating the laser light described above.

After irradiating the laser light, another heat treatment is performedwithin an oxidizing atmosphere to which HCl is added to form a thermaloxide film on the surface of the crystalline silicon film.

At this time, the metal element is gettered to the thermal oxide film bythe action of chlorine and the concentration of the metal element withinthe crystalline silicon film is reduced. Further, the nickel element isgasified and removed to the outside by the action of chlorine.

The heat treatment for gettering the nickel element is preferable toperform at a temperature higher than that of the heat treatment for thecrystallization. It is preferable to perform the heat treatment at atemperature over 600° C., or more preferably at 640° C. or more. Theupper limit thereof may be adequately set as temperature below 750° C.

As a result of the heat treatment for gettering, a thermal oxide filmcontaining the nickel element in high concentration is formed. Then, thecrystalline silicon film having the high crystallinity and having lowconcentration of the metal element may be obtained by removing thisthermal oxide film.

The use of the invention disclosed in the present specification allowsto provide the technology for reducing the concentration of metalelement within the crystalline silicon film which has been obtained byutilizing the metal element which promotes the crystallization ofsilicon. The use of this technology also allows a more reliable andhigher performance thin film semiconductor device to be obtained.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows steps for obtaining a crystalline silicon film.

FIG. 2 shows steps for obtaining a crystalline silicon film.

FIG. 3 shows steps for fabricating a thin film transistor.

FIG. 4 shows steps for fabricating a thin film transistor.

FIG. 5 shows steps for fabricating a thin film transistor.

FIG. 6 shows steps for fabricating an active layer of the thin filmtransistor.

FIG. 7 shows states when laser light is irradiated to patterns made ofthe crystalline silicon film.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS First Embodiment

An arrangement for obtaining a crystalline silicon film on a glasssubstrate by utilizing nickel element will be explained in the presentembodiment.

At first, the crystalline silicon film having a high crystallinity isobtained by an action of nickel element in the present embodiment.

Then, laser light is irradiated to enhance the crystallinity of the filmand to diffuse nickel element existing locally concentrated within thefilm. That is, the block of nickel is extinguished.

Next, a thermal oxide film containing a halogen element is formed on thecrystalline silicon film by thermal oxidation. At this time, the nickelelement remaining in the crystalline silicon film thus obtained isgettered to the thermal oxide film by an action of the halogen element.Because the nickel element is distributed by the irradiation of thelaser light, the gettering proceeds effectively.

Then, the thermal oxide film containing the nickel element in highconcentration as a result of the gettering is removed. Thereby, thecrystalline silicon film having nickel element in low concentrationwhile having the high crystallinity is obtained on the glass substrate.

A fabrication process of the present embodiment will be explained byusing FIG. 1. At first, a silicon oxide nitride film 102 is formed as anunderlying film in a thickness of 3000 angstrom on the glass substrate101 of Corning 1737 (distortion point: 667° C.).

The silicon oxide nitride film is formed by using plasma CVD usingsiliane, N₂O gas and oxygen as original gases. Or, it may be formed byusing plasma CVD using TEOS gas and N₂O gas.

The silicon oxide nitride film has a function of suppressing thediffusion of impurities from the glass substrate in the later steps(seeing from the level of fabrication of a semiconductor, a glasssubstrate contains a large amount of impurities).

It is noted that the silicon nitride film is most suitable to maximizethe function of suppressing the diffusion of the impurities. However,the silicon nitride film is not practical because it may be peeled offfrom the glass substrate due to the influence of stress. A silicon oxidefilm may be also used as the underlying film instead of the siliconoxide nitride film.

It is also important to increase the hardness of the underlying film 102as much as possible. It is concluded from the fact that the harder thehardness of the underlying film (i.e. the smaller the etching ratethereof), the higher the reliability is in an endurance test of the thinfilm transistor obtained finally. It is assumed to be caused by theeffect of blocking the impurities from the glass substrate in thefabrication process of the thin film transistor.

It is also effective to include a small amount of halogen elementtypified by chlorine in the underlying film 102. Thereby, the metalelement which promotes crystallization of silicon and which existswithin the semiconductor layer may be gettered by the halogen element inthe later step.

It is also effective to perform a hydrogen plasma treatment afterforming the underlying film. It is also effective to perform a plasmatreatment in an atmosphere in which oxygen and hydrogen are mixed. Thesetreatments are effective in removing carbon component which is adsorbedon the surface of the underlying film and in enhancing the interfacialcharacteristic with a semiconductor film formed later.

Next, an amorphous silicon film 103, which turns out to be a crystallinesilicon film later, is formed in a thickness of 500 angstrom by lowpressure thermal CVD. The reason why low pressure thermal CVD is used isbecause the quality of the crystalline silicon film obtained later isbetter, i.e. the film quality is denser. Besides the low pressurethermal CVD, the plasma CVD may be used.

The amorphous silicon film fabricated here is desirable to have 5×10¹⁷cm⁻³ to 2×10¹⁹ cm⁻³ of concentration of oxygen within the film. It isbecause oxygen plays an important role in the later step of getteringthe metal element (which promotes crystallization of silicon).

However, it must be careful here because the crystallization of theamorphous silicon film is hampered if the oxygen concentration is higherthan the above-mentioned range of concentration.

The concentration of other impurities such as those of nitrogen andcarbon is preferred to be as low as possible. In specifically, theconcentration must be below 2×10¹⁹ cm⁻³.

The upper limit of the thickness of the amorphous silicon film is about2000 angstrom. It is because it is disadvantageous to have a thick filmto obtain the effect of laser irradiated later. Thick film isdisadvantageous because the laser light irradiated to the silicon filmis absorbed almost completely at the surface of the film.

The lower limit of the amorphous silicon film 103 is practically about200 angstrom, though it depends on how it is formed.

Next, nickel element is introduced to the amorphous silicon film 103 tocrystallize it. Here, the nickel element is introduced by applyingnickel acetate solution containing 10 ppm (to weight) of nickel on thesurface of the amorphous silicon film 103.

Besides the method of using the above-mentioned solution, sputtering,CVD, plasma treatment or adsorption may be used as the method forintroducing the nickel element.

The method of using the solution is useful in that it is simple and thatthe concentration of the metal element may be readily adjusted.

The nicker acetate solution is applied as described above to form aliquid film 104 as shown in FIG. 1A. After obtaining this state, extrasolution is blown out by using a spin coater not shown. Thus, the nickelelement is held in contact on the surface of the amorphous silicon film103.

It is noted that it is preferable to use nickel sulfate solution,instead of using the nickel acetate, if the remained impurities in thelater heating process is taken into consideration. It is because thenickel acetate contains carbon and it might be carbonized in the laterheating process, thus remaining within the film.

An amount of the nickel element to be introduced may be controlled byadjusting the concentration of nickel element within the solution.

Next, a heat treatment is performed in the temperature range from 550°C. to 650° C. in the state shown in FIG. 1B to crystallize the amorphoussilicon film 103 and to obtain a crystalline silicon film 105. This heattreatment is performed in a reducing atmosphere.

It is preferable to perform the heat treatment below the temperature ofthe distortion point of the glass substrate. Because the distortionpoint of the Corning 1737 glass substrate is 667° C., the upper limit ofthe heating temperature here is preferable to be about 650° C., leavingsome margin.

Here, the heat treatment is performed for four hours at 620° C. within anitrogen atmosphere containing 3% of hydrogen.

The reason why the reducing atmosphere is adopted in the crystallizationstep by way of the heat treatment is to prevent oxides from beingcreated in the step of the heat treatment and more concretely, tosuppress nickel from reacting with oxygen and NiOx from being created onthe surface of the film or within the film.

Oxygen couples with nickel and contributes a lot in gettering nickel inthe later gettering step. However, it has been found that if oxygencouples with nickel in the above-mentioned stage of the crystallization,it hampers the crystallization. Accordingly, it is important to suppressthe oxides from being created to the utmost in the crystallization stepcarried out by way of heating.

The concentration of oxygen within the atmosphere for performing theheat treatment for the crystallization has to be in an order of ppm, orpreferably, less than 1 ppm.

Inert gases such as argon, besides nitrogen, may be used as the gaswhich occupies the most of the atmosphere for performing the heattreatment for the crystallization.

After the crystallization step by way of the heat treatment, nickelelement remains as blocks. This fact has been confirmed from theobservation by means of TEM (transmission electron microscope).

Although the cause of the fact that the nickel exists as blocks is notclear yet, it is considered to be related with some crystallizationmechanism.

Next, laser light is irradiated as shown in FIG. 1C. Here, KrF excimerlaser (wavelength: 248 nm) is used. Here, a method of irradiating thelaser light by scanning its linear beam is adopted.

The nickel element which has been locally concentrated as a result ofthe crystallization carried out by way of the aforementioned heattreatment is distributed by a certain degree within the film 105 byirradiating the laser light. That is, the nickel element may bedistributed by disappearing. the blocks of the nickel element.

Another heat treatment is performed in the step shown in FIG. 1D to forma thermal oxide film for gettering the nickel element. Here, this heattreatment is performed within an atmosphere containing halogen element.Specifically, the heat treatment is carried out in an oxygen atmospherecontaining 5% of HCl (FIG. 1D).

This step is carried out to remove the nickel element (or anotherelement which promotes crystallization of silicon) which has beenintroduced intentionally for the crystallization in the initial stagefrom the crystalline silicon film 105. This heat treatment is performedat a temperature higher than that of the heat treatment performed forthe crystallization described above. It is an important condition foreffectively performing the gettering of nickel element.

This heat treatment is performed in the temperature range from 600° C.to 750° C. upon meeting the above-mentioned condition. The effect ofgettering the nickel element in this step may be obtained remarkablywhen the temperature is higher than 600° C.

In this step, the nickel element which has been distributed by theabove-mentioned irradiation of laser is effectively gettered to theoxide film.

Further, the upper limit of the heating temperature is limited by thedistortion point of the glass substrate to be used. It must be carefulnot to perform the heat treatment in a temperature above the distortionpoint of the glass substrate to be used because it is deformed.

It is preferable to mix HCl with a ratio of 0.5% to 10% (volume %) ofoxygen. It must be careful not to mix it above this concentrationbecause, otherwise, the surface of the film is roughened with the samedegree of irregularity with the thickness of the film.

By performing the heat treatment under such conditions, the thermaloxide film 106 containing chlorine as shown in FIG. 1D is formed. Here,the heat treatment is performed for 12 hours and the thickness of thethermal oxide film 106 is 100 angstrom.

Because the thermal oxide film 106 is formed, the thickness of thecrystalline silicon film 103 becomes about 450 angstrom.

When the heating temperature is 600° C. to 750° C. in this heattreatment, the treatment time (heating time) is set at 10 hours to 48hours, typically at 24 hours.

This treatment time may be set adequately depending on the thickness ofthe oxide film to be obtained as a matter of course.

In this step, nickel element is gettered out of the silicon film by theaction of the halogen element. Here, the nickel element is gettered tothe thermal oxide film 106 formed by the action of chlorine.

In the gettering, oxygen existing within the crystalline silicon filmplays an important role. That is, the gettering of the nickel elementproceeds effectively by chlorine acting on nickel oxide formed whenoxygen couples with nickel.

If the concentration of oxygen is too much, it becomes the factor ofhampering the crystallization of the amorphous silicon film 103 in thecrystallization step shown in FIG. 1B as described above. However, theexistence thereof plays an important role in the process of getteringnickel as described above. Accordingly, it is important to control theconcentration of oxygen existing within the amorphous silicon film as astarting film.

Here, Cl has been selected as the halogen element and the case of usingHCl has been shown as a method for introducing it. Besides HCl, one typeor a plurality of types of mixed gas selected from HF, HBr, Cl₂, F₂, Br₂may be used. Besides them, halogen hydroxide may be used in general.

It is preferable to set the content (volume content) of those gaseswithin the atmosphere to 0.25 to 5% if it is HF, 1 to 15% if it is HBr,0.25 to 5% if it is Cl₂, 0.125 to 2.5% if it is F₂ and 0.5 to 10% if itis Br₂.

If the concentration is below the above-mentioned range, no significanteffect is obtained. Further, if the concentration exceeds the upperlimit of the above-mentioned range, the surface of the crystallinesilicon film is roughened.

Through this step, the concentration of nickel element may be reduced toless than 1/10 of the maximum from the initial stage. It means that thenickel element may be reduced to 1/10 as compared to the case when nogettering by the halogen element is conducted. This effect may beobtained in the same manner even when another metal element is used.

Because the nickel element is gettered to the oxide film formed in theabove-mentioned step, naturally the nickel concentration within theoxide film becomes high as compared to other regions.

Further, it has been observed that the concentration of nickel elementis apt to be high near the interface of the crystalline silicon film 105with the oxide film 106. It is considered to happen because the regionwhere the gettering mainly takes place is on the side of the oxide filmnear the interface between the silicon film and the oxide film. Thegettering proceeding near the interface is considered to be caused bythe existence of stress and defects.

Then, the thermal oxide film 106 containing nickel in high concentrationis removed. The thermal oxide film 106 may be removed by means of wetetching or dry etching using buffered hydrofluoric acid (or otherhydrofluorite etchant).

Thus, a crystalline silicon film 107 in which the concentration ofnickel has been reduced is obtained as shown in FIG. 1E.

Because nickel element is contained near the surface of the obtainedcrystalline silicon film 107 in relatively high concentration, it iseffective to advance the above-mentioned etching to over-etch, more orless, the surface of the crystalline silicon film 107.

It is also effective to irradiate laser light again after removing thethermal oxide film 106 to promote the crystallinity of the crystallinesilicon film 107 thus obtained further. That is, it is effective toirradiate laser light again after gettering the nickel element.

Although the case when the KrF excimer laser (wavelength: 248 nm) isused as the laser to be used has been shown in the presentspecification, it is possible to use a XeCl excimer laser (wavelength:308 nm) and other types of excimer lasers.

It is also possible to arrange so as to irradiate ultraviolet rays orinfrared rays for example instead of laser light.

Second Embodiment

The present embodiment relates to a case when Cu is used as the metalelement which promotes the crystallization of silicon in the arrangementshown in the first embodiment. In this case, cupric acetate [Cu(CH₃COO)₂] or cupricchloride (CuCl₂2H₂O) may be used as the solution forintroducing Cu.

Third Embodiment

The present embodiment relates to a case of growing crystal in the formdifferent from that in the first embodiment. That is, the presentembodiment relates to a method of growing the crystal in a directionparallel to the substrate, i.e. a method called lateral growth, byutilizing the metal element which promotes crystallization of silicon.

FIG. 2 shows the fabrication process according to the presentembodiment. At first, a silicon oxide nitride film is formed as anunderlying film 202 in a thickness of 3000 angstrom on the Corning 1737glass substrate (or a quartz substrate) 201.

Next, an amorphous silicon film 203 which is the starting film for acrystalline silicon film is formed in a thickness of 600 angstrom by lowpressure thermal CVD. The thickness of the amorphous silicon film ispreferable to be less than 2000 angstrom as described before.

It is noted that plasma CVD may be used instead of the low pressurethermal CVD.

Next, a silicon oxide film not shown is formed in a thickness of 1500angstrom and is patterned to form a mask 204. An opening is created onthe mask in a region 205. The amorphous silicon film 203 is exposed atthe region where the opening 205 is created.

The opening 205 has a thin and long rectangular shape in thelongitudinal direction from the depth to the front side of the figure.Preferably, the width of the opening 203 is 20 μm or more. The lengththereof in the longitudinal direction may be determined arbitrarily.

Then, the nickel acetate aqueous solution containing 10 ppm of nickelelement in terms of weight is applied in the same manner with the firstembodiment and the extra solution is removed by performing spin dryingby using a spinner not shown.

Thus, the solution is held in contact on the exposed surface of theamorphous silicon film 203 as indicated by a dotted line 206 in FIG. 2A.

Next, a heat treatment is performed at 640° C. for four hours in anitrogen atmosphere containing 3% of hydrogen and in which oxygen isminimized. Then, crystal grows in the direction parallel to thesubstrate as indicated by the reference numeral 207 in FIG. 2B. Thiscrystal growth advances from the region of the opening 205 to whichnickel element has been introduced to the surrounding part. This crystalgrowth in the direction parallel to the substrate will be referred to aslateral growth.

It is possible to advance this lateral growth across more than 100 μmunder the conditions shown in the present embodiment. Then, a siliconfilm 208 having a region in which the crystal has thus grown laterallyis obtained. It is noted that crystal growth in the vertical directioncalled vertical growth advances from the surface of the silicon film tothe underlying interface in the region where the opening 205 is formed.

Then, the mask 204 made of the silicon oxide film for selectivelyintroducing nickel element is removed. Thus, the state shown in FIG. 2Cis obtained. In this state, the vertical growth region, the lateralgrowth region and a region in which no crystal has grown (havingamorphous state) exist within the silicon film 208.

In this state, the nickel element is unevenly distributed in the film.In particular, the nickel element exists in relatively highconcentration at the region where the opening 205 has been formed and atthe edge portion 207 of the crystal growth.

After obtaining the state shown in FIG. 2C, laser light is irradiated.The KrF excimer laser is irradiated here similarly to the firstembodiment.

The nickel element which has been unevenly distributed is diffused inthis step to obtain a condition in which it can be gettered readily inthe later gettering step.

After irradiating the laser light, a heat treatment is performed at 650°C. for 12 hours within an oxygen atmosphere containing 3% of HCl. Inthis step, an oxide film 209 containing nickel element in highconcentration is formed. In the same time, the concentration of nickelelement within the silicon film 208 may be reduced relatively (FIG. 2D).

Here, the thermal oxide film 209 is formed in a thickness of 100angstrom. The thermal oxide film contains the nickel element thusgettered by the action of chlorine in high concentration. Further,because the thermal oxide film 209 is formed, the thickness of thecrystalline silicon film 208 is reduced to about 500 angstrom.

Next, the thermal oxide film 209 containing nickel element in highconcentration is removed.

In the crystalline silicon film of this state, the nickel element has adistribution of concentration such that it exists in high concentrationtoward the surface of the crystalline silicon film. This state is causedby the fact that the nickel element has been gettered to the thermaloxide film 209 when the thermal oxide film was formed.

Accordingly, it is useful to etch the surface of the crystalline siliconfilm to remove the region in which the nickel element exists in highconcentration after removing the thermal oxide film 209. That is, thecrystalline silicon film in which the nickel element concentration isreduced further may be obtained by etching the surface of thecrystalline silicon film in which the nickel element exists in highconcentration. However, it is necessary to consider the thickness of thesilicon film finally obtained at this time.

Next, patterning is performed to form a pattern 210 formed of thelaterally grown region.

The concentration of nickel element which remains within the pattern 210made of the lateral growth region thus obtained may be reduced furtheras compared to the case shown in the first embodiment.

This is caused by the fact that the concentration of the metal elementcontained within the lateral growth region is low originally.Specifically, the concentration of nickel element within the pattern 209made of the lateral growth region may be readily reduced to the order of10¹⁷ cm⁻³ or less.

When a thin film transistor is formed by utilizing the lateral growthregion, a semiconductor device having a higher mobility may be obtainedas compared to the case when the vertical growth region as shown in thefirst embodiment (crystal grows vertically on the whole surface in thecase of the first embodiment) is utilized.

It is noted that it is useful to perform the etching process furtherafter forming the pattern shown in FIG. 2E to remove the nickel elementexisting on the surface of the pattern.

Then, a thermal oxide film 211 is formed after forming the pattern 210.This thermal oxide film is formed into a thickness of 200 angstrom byperforming a heat treatment for 12 hours in an oxygen atmosphere at 650°C.

This thermal oxide film becomes a part of a gate insulating film laterwhen a thin film transistor is constructed.

If the thin film transistor is to be fabricated thereafter, a siliconoxide film is formed further by means of plasma CVD or the like so as tocover the thermal oxide film 211 to form a gate insulating film.

Fourth Embodiment

The present embodiment relates to a case of fabricating a thin filmtransistor disposed in a pixel region of an active matrix type liquidcrystal display or an active matrix type EL display by utilizing theinvention disclosed in the present specification.

FIG. 3 shows the fabrication process according to the presentembodiment. At first, the crystalline silicon film is formed on theglass substrate through the process shown in the first or the thirdembodiment. When the crystalline silicon film is obtained by thearrangement shown in the first embodiment, it is patterned to obtain thestate shown in FIG. 3A.

In the state shown in FIG. 3A, the reference numeral 301 denotes a glasssubstrate, 302 an underlying film, and 303 an active layer formed of thecrystalline silicon film. After obtaining the state shown in FIG. 3A, aplasma treatment is performed within a reduced pressure atmosphere inwhich oxygen and hydrogen are mixed. The plasma is generated byhigh-frequency discharge.

Organic substances existing on the surface of the exposed active layer303 may be removed by the above-mentioned plasma treatment. To be exact,the organic substances adsorbing on the surface of the active layer areoxidized by oxygen plasma and the oxidized organic substances arereduced and vaporized further by hydrogen plasma. Thus the organicsubstances existing on the surface of the exposed active layer 303 areremoved.

The removal of the organic substances is very effective in suppressingfixed charge from existing on the surface of the active layer 303.Because the fixed charge caused by the existence of organic substanceshampers the operation of the device and renders the characteristicsthereof instable, it is very useful to reduce it.

After removing the organic substances, thermal oxidation is performedwithin an oxygen atmosphere at 640° C. to form a thermal oxide film 300of 100 angstrom thick. This thermal oxide film has a high interfacialcharacteristic with a semiconductor layer and composes a part of a gateinsulating film later. Thus, the state shown in FIG. 3A is obtained.

After obtaining the state shown in FIG. 3A, a silicon oxide nitride film304 which composes the gate insulating film is formed in a thickness of1000 angstrom. The film may be formed by using plasma CVD using mixedgas of oxygen, siliane and N₂O or plasma CVD using mixed gas of TEOS andN₂O.

The silicon oxide nitride film 304 functions as the gate insulating filmtogether with the thermal oxide film 300.

It is also effective to contain halogen element within the silicon oxidenitride film. That is, by fixing the nickel element by the action of thehalogen element, it is possible to prevent the function of the gateinsulating film as an insulating film from being reduced by theinfluence of the nickel element (or another metal element which promotescrystallization of silicon) existing within the active layer.

It is significant to use the silicon oxide nitride film in that themetal element hardly infiltrates to the gate insulating film because ofits dense film quality. If the metal element infiltrates into the gateinsulating film, its function as an insulating film is reduced, thuscausing instability and dispersion of characteristics of the thin filmtransistor.

It is noted that a silicon oxide film which is normally used may be alsoused for the gate insulating film.

After forming the silicon oxide nitride film 304 which functions as thegate insulating film, an aluminum film not shown which functions as agate electrode later is formed by sputtering. Scandium is included 0.2weight % of aluminum within the aluminum film.

Scandium is included in the aluminum film to suppress hillock andwhisker from being generated in the later process. The hillock andwhisker mean that abnormal growth of aluminum occurs by heating, thusforming needle or prickle-like projections.

After forming the aluminum film, a dense anodic oxide film not shown isformed. The anodic oxide film is formed by using ethylene glycolsolution containing 3% of tartaric acid as electrolyte. That is, theanodic oxide film having the dense film quality is formed on the surfaceof the aluminum film by setting the aluminum film as the anode andplatinum as the cathode and by anodizing within this electrolyte.

The thickness of the anodic oxide film not shown having the dense filmquality is around 100 angstrom. This anodic oxide film plays a role ofenhancing the adhesiveness with a resist mask to be formed later.

It is noted that the thickness of the anodic oxide film may becontrolled by adjusting voltage applied during the anodization.

Next, the resist mask 306 is formed and the aluminum film is patternedso as to have a pattern 305. The state shown in FIG. 3B is thusobtained.

Here, another anodization is performed. In this case, 3% of oxalateaqueous solution is used as electrolyte. A porous anodic oxide film 308is formed by anodizing within this electrolyte by setting the aluminumpattern 305 as the anode.

In this step, the anodic oxide film 308 is formed selectively on thesides of the aluminum pattern because the resist mask 306 having thehigh adhesiveness exists thereabove.

The anodic oxide film may be grown up to several/m thick. The thicknessis 6000 angstrom here. It is noted that the range of the growth may becontrolled by adjusting an anodizing time.

Next, the resist mask 306 is removed. Then, a dense anodic oxide film isformed again. That is, the anodization is performed again by using theethylene glycol solution containing 3% of tartaric acid as electrolyte.Then, an anodic oxide film 309 having a dense film quality is formedbecause the electrolyte infiltrates into the porous anodic oxide film308.

This dense anodic oxide film 309 is 1000 angstrom thick. The thicknessis controlled by adjusting applied voltage.

Here, the exposed silicon oxide nitride film 304 and the thermal oxidefilm 300 are etched by utilizing dry etching. Then, the porous anodicoxide film 308 is removed by using mixed acid in which acetic acid,nitric acid and phosphoric acid are mixed. Thus, the state shown in FIG.3D is obtained.

After obtaining the state shown in FIG. 3D, impurity ions are implanted.Here, P (phosphorus) ions are implanted by plasma doping in order tofabricate an N-channel type thin film transistor.

In this step, heavily doped regions 311 and 315 and lightly dopedregions 312 and 314 are formed because part of the remaining siliconoxide film 310 functions as a semi-permeable mask, thus blocking part ofthe implanted ions.

Then, laser light or intense light is irradiated to activate the regionsinto which the impurity ions have been implanted. Thus, a source region311, a channel forming region 313, a drain region 315 and lowconcentration impurity regions 312 and 314 are formed in a manner ofself-alignment.

One designated by the reference numeral 314 here is the region calledthe LDD (lightly doped region) (FIG. 3D).

It is noted that when the dense anodic oxide film 309 is formed as thickas 2000 angstrom or more, offset gate regions may be formed on theoutside of the channel forming region 313 by its thickness.

Although the offset gate regions are formed also in the presentembodiment, it is not shown in the figures because its size is small,its contribution due to the existence thereof is small and because thefigures might otherwise become complicated.

Next, a silicon oxide film or a silicon nitride film or their laminatedfilm is formed as an interlayer insulating film 316. The interlayerinsulating film may be constructed by forming a layer made of a resinmaterial on the silicon oxide film or the silicon nitride film.

Then, contact holes are created to form a source electrode 317 and adrain electrode 318. Thus, the thin film transistor shown in FIG. 3E iscompleted.

Fifth Embodiment

The present embodiment relates to a method for forming the gateinsulating film 304 in the arrangement shown in the fourth embodiment.Thermal oxidation may be used as a method for forming the gateinsulating film when a quartz substrate or a glass substrate having ahigh heat resistance is used as the substrate.

The thermal oxidation allows the film quality to be densified and isuseful in obtaining a thin film transistor having stablecharacteristics.

That is, because an oxide film formed by the thermal oxidation is denseas an insulating film and movable electric charge existing therein canbe reduced, it is one of the most suitable film as a gate insulatingfilm.

As the method for forming the thermal oxide film, a heat treatmentperformed in an oxidizing atmosphere at 950° C. may be cited.

At this time, it is effective to mix HCl or the like within theoxidizing atmosphere. Thereby, the metal element existing within theactive layer may be fixed in the same time with the formation of thethermal oxide film.

It is also effective to form a thermal oxide film containing nitrogencomponent by mixing N₂O gas within the oxidizing atmosphere. Here, it ispossible to obtain a silicon oxide nitride film by the thermal oxidationby optimizing the mixed ratio of the N₂O gas.

It is noted that the thermal oxide film 300 needs not be always formedin the present embodiment.

Sixth Embodiment

The present embodiment relates to a case of fabricating a thin filmtransistor through a process different from that shown in FIG. 3.

FIG. 4 shows the fabrication process according to the presentembodiment. At first, the crystalline silicon film is formed on theglass substrate through the process shown in the first or thirdembodiment. It is then patterned, thus obtaining the state shown in FIG.4A.

After obtaining the state shown in FIG. 4A, a plasma treatment isperformed within a reduced pressure atmosphere in which oxygen andhydrogen are mixed.

In the state shown in FIG. 4A, the reference numeral 401 denotes a glasssubstrate, 402 an underlying film, 403 an active layer made of thecrystalline silicon film and 400 a thermal oxide film formed again afterremoving the thermal oxide film for gettering.

After obtaining the state shown in FIG. 4A, a silicon oxide nitride film404 which composes a gate insulating film is formed in a thickness of1000 angstrom. The film may be formed by using plasma CVD using mixedgas of oxygen, siliane and N₂O or plasma CVD using mixed gas of TEOS andN₂O.

The silicon oxide nitride film 404 composes the gate insulating filmtogether with the thermal oxide film 400. It is noted that a siliconoxide film may be used besides the silicon oxide nitride film.

After forming the silicon oxide nitride film 404 which functions as thegate insulating film, an aluminum film (not shown) which functions as agate electrode later is formed by sputtering. Scandium is includedwithin the aluminum film at 0.2 weight %.

After forming the aluminum film, a dense anodic oxide film not shown isformed. The anodic oxide film is formed by using ethylene glycolsolution containing 3% of tartaric acid as electrolyte. That is, theanodic oxide film having the dense film quality is formed on the surfaceof the aluminum film by setting the aluminum film as the anode andplatinum as the cathode and by anodizing within this electrolyte.

The thickness of the anodic oxide film not shown having the dense filmquality is about 100 angstrom. This anodic oxide film plays a role ofenhancing the adhesiveness with a resist mask to be formed later.

It is noted that the thickness of the anodic oxide film may becontrolled by adjusting voltage applied during the anodization.

Next, the resist mask 405 is formed and the aluminum film is patternedso as to have a pattern 406.

Here, another anodization is performed. In this case, 3% of oxalateaqueous solution is used as electrolyte. A porous anodic oxide film 407is formed by anodizing within this electrolyte by setting the aluminumpattern 406 as the anode.

In this step, the anodic oxide film 407 is formed selectively on thesides of the aluminum pattern because the resist mask 405 having thehigh adhesiveness exists thereabove.

The anodic oxide film may be grown up to several μm thick. The thicknessis 6000 angstrom here. It is noted that the range of the growth may becontrolled by adjusting an anodizing time.

Then, the resist mask 405 is removed and another dense anodic oxide filmis formed. That is, the anodization is performed again by using theethylene glycol solution containing 3% of tartaric acid as electrolyte.Then, an anodic oxide film 408 having a dense film quality is formedbecause the electrolyte infiltrates into the porous anodic oxide film407 (FIG. 2C).

Here, the initial implantation of impurity ions is performed. This stepmay be performed after removing the resist mask 405.

A source region 409 and a drain region 411 are formed by implanting theimpurity ions. No impurity ion is implanted to a region 410.

Then, the porous anodic oxide film 407 is removed by using mixed acid inwhich acetic acid, nitric acid and phosphoric acid are mixed. Thus, thestate shown in FIG. 4D is obtained.

After obtaining the state shown in FIG. 4D, impurity ions are implantedagain. The impurity ions are implanted under the doping conditionlighter than that of the first implantation.

In this step, lightly doped regions 412 and 413 are formed and a region414 turns out to be a channel forming region (FIG. 4D).

Then, laser light or intense light is irradiated to activate the regionsinto which the impurity ions have been implanted. Thus, the sourceregion 409, the channel forming region 414, the drain region 411 and lowconcentration impurity regions 412 and 413 are formed in a manner ofself-alignment.

Here, one designated by the reference numeral 413 is the region calledthe LDD (lightly doped region) (FIG. 4D).

Next, a silicon oxide film or a silicon nitride film or their laminatedfilm is formed as an interlayer insulating film 414. The interlayerinsulating film may be constructed by forming a layer made from a resinmaterial on the silicon oxide film or the silicon nitride film.

After that, contact holes are created to form a source electrode 416 anda drain electrode 417. Thus, the thin film transistor shown in FIG. 4Eis completed.

Seventh Embodiment

The present embodiment relates to a case when an N-channel type thinfilm transistor and a P-channel type thin film transistor are formed ina complementary manner.

The arrangement shown in the present embodiment may be utilized forvarious thin film integrated circuits integrated on an insulated surfaceas well as for peripheral driving circuits of an active matrix typeliquid crystal display for example.

At first, a silicon oxide film or a silicon oxide nitride film is formedas an underlying film 502 on a glass substrate 501 as shown in FIG. 5A.It is preferable to use the silicon oxide nitride film.

Next, an amorphous silicon film not shown is formed by plasma CVD or lowpressure thermal CVD. Then, the amorphous silicon film is transformedinto a crystalline silicon film by the same method as shown in the firstembodiment.

Next, a plasma treatment is performed within an atmosphere in whichoxygen and hydrogen are mixed. Then, the obtained crystalline siliconfilm is patterned to obtain active layers 503 and 504. Thus, the stateshown in FIG. 5A is obtained.

It is noted that a heat treatment of ten hours at 650° C. is performedwithin a nitrogen atmosphere containing 3% of HCl in the state shown inFIG. 5A in order to suppress the influence of carriers moving the sideof the active layer.

Because an OFF current characteristic is degraded if a trap level existson the side of the active layer due to the existence of the metalelement, it is useful, by performing the process shown here, to reducethe density of the level on the side of the active layer.

Further, the thermal oxide film 500 and the silicon oxide nitride film505 which compose a gate insulating film are formed. If quartz is usedas the substrate here, it is desirable to compose the gate insulatingfilm only by the thermal oxide film formed by using the above-mentionedthermal oxidation.

Next, an aluminum film not shown which composes a gate electrode lateris formed in a thickness of 4000 angstrom. Besides the aluminum film, ametal which can be anodized (e.g. tantalum) may be used.

After forming the aluminum film, a very thin and dense anodic oxide filmis formed on the surface thereof by the method described before.

Next, a resist mask not shown is provided on the aluminum film topattern the aluminum film. Then, anodization is performed by setting theobtained aluminum pattern as the anode to form porous anodic oxide films508 and 509. The thickness of the porous anodic oxide films is 5000angstrom for example.

Then, another anodization is performed under the condition of formingdense anodic oxide films to form dense anodic films 510 and 511. Thethickness of the dense anodic oxide films 510 and 511 is 800 angstrom.Thus, the state shown in FIG. 5B is obtained.

Then, the exposed silicon oxide film 505 and the thermal oxide film 500are removed by dry etching, thus obtaining the state shown in FIG. 5C.

After obtaining the state shown in FIG. 5C, the porous anodic oxidefilms 508 and 509 are removed by using mixed acid in which acetic acid,nitric acid and phosphoric acid are mixed: Thus, the state shown in FIG.5D is obtained.

Here, resist masks are disposed alternately to implant P ions to thethin film transistor on the left side and B ions to the thin filmtransistor on the right side.

By implanting those impurity ions, a source region 514 and a drainregion 517 having N-type in high concentration are formed in a manner ofself-alignment.

Further, a region 515 to which P ions are doped in low concentration,thus having weak N-type, as well as a channel forming region 516 areformed in the same time.

The reason why the region 515 having the weak N-type is formed isbecause the remaining gate insulating film 512 exists. That is, part ofP ions transmitting through the gate insulating film 512 is blocked bythe gate insulating film 512.

By the same principle, a source region 521 and a drain region 518 havingstrong P-type are formed in a manner of self-alignment and a lowconcentrate impurity region 520 is formed in the same time. Further, achannel forming region 519 is formed in the same time.

In the case that when the thickness of the dense anodic oxide films 510and 511 is as thick as 2000 angstrom, an offset gate region may beformed in contact with the channel forming region by that thickness.

Its existence may be ignored in the case of the present embodimentbecause the dense anodic oxide films 510 and 511 are so thin as lessthan 1000 angstrom.

Then, laser light or intense light is irradiated to anneal the regioninto which the impurity ions have been implanted.

Then, a silicon nitride film 522 and a silicon oxide film 523 are formedas interlayer insulating films as shown in FIG. 5E. The thickness is1000 angstrom, respectively. It is noted that the silicon oxide film 523need not be formed.

Here, the thin film transistor is covered by the silicon nitride film.The reliability of the thin film transistor may be enhanced by arrangingas described above because the silicon nitride film is dense and has afavorable interfacial characteristic.

Further, an interlayer insulating film 524 made of a resin material isformed by means of spin coating. Here, the thickness of the interlayerinsulating film 524 is 1 μm (FIG. 5E).

Then, contact holes are created to form a source electrode 525 and adrain electrode 526 of the N-channel type thin film transistor on theleft side. In the same time, a source electrode 527 and the drainelectrode 526 of the thin film transistor on the right side are formed.Here, the electrode 526 is disposed in common.

Thus, the thin film transistor circuit having a CMOS structureconstructed in a complementary manner may be formed.

According to the arrangement shown in the present embodiment, the thinfilm transistor is covered by the nitride film and further the resinmaterial. This arrangement allows to enhance the durability of the thinfilm transistor to which movable ions nor moisture hardly infiltrate.

Further, it allows to prevent capacitance from being generated betweenthe thin film transistor and wires when a multi-layered wire is formed.

Eighth Embodiment

The present embodiment relates to a case when nickel element isintroduced directly to the surface of the underlying film in the processshown in the first embodiment. In this case, the nickel element is heldin contact with the lower surface of the amorphous silicon film.

In this case, nickel element is introduced after forming the underlyingfilm such that the nickel element (metal element) is held in contactwith the surface of the underlying film. Besides the method of using thesolution, sputtering, CVD or adsorption may be used as the method forintroducing nickel element.

Ninth Embodiment

The present embodiment is characterized in that the crystallinity of anisland pattern formed of a crystalline silicon film in the state shownin FIG. 2E, the state shown in FIG. 3A or the state shown in FIG. 4A isimproved by irradiating laser light.

A predetermined annealing effect can be obtained with relatively lowirradiation energy density by irradiating the laser light in the stateshown in FIGS. 2E, 3A and 4A.

It is considered to have been effected because the laser energy isirradiated to a spot of small area, thus enhancing the efficiency ofenergy utilized in the annealing.

Tenth Embodiment

The present embodiment relates to a case in which patterning of anactive layer of a thin film transistor is devised in order to enhancethe effect of annealing by the irradiation of laser light.

FIG. 6 shows a process for fabricating the thin film transistoraccording to the present embodiment. At first, a silicon oxide film orsilicon oxide nitride film is formed as an underlying layer on a Corning1737 glass substrate 601.

Next, an amorphous silicon film not shown is formed in a thickness of500 angstrom by using low pressure thermal CVD. It is noted that thisamorphous silicon film turns out to be a crystalline silicon film 603through the crystallization process later.

Next, the amorphous silicon film not shown is crystallized by the methodshown in the first embodiment (see FIG. 1) or third embodiment (see FIG.2) to obtain the crystalline silicon film. Thus, the state shown in FIG.6A is obtained.

After obtaining the state shown in FIG. 6A, the crystalline silicon film603 is formed on the glass substrate in accordance to the process shownin the first embodiment whose fabrication process is shown in FIG. 1 orthe third embodiment whose fabrication process is shown in FIG. 2. Thatis, the amorphous silicon film is crystallized by the heat treatmentusing nickel element to obtain the crystalline silicon film 604. Theheat treatment is performed at 620° C. for four hours.

After obtaining the crystalline silicon film, a pattern for constructingan active layer of a thin film transistor is formed. At this time, thepattern is formed so as to have a sectional profile 604 shown in FIG.6B.

The pattern 604 as shown in FIG. 6B is formed in order to suppress theshape of the pattern from being deformed in the later treatment step ofirradiating laser light.

In general, when laser light is irradiated to a pattern 702 made of anormal island-shape silicon film formed on a base 701 as shown in FIG.7A, a convex portion 704 is formed at the edge of a pattern 703 afterthe irradiation of the laser light as shown in FIG. 7B.

It is considered to happen because energy of the irradiated laser lightis concentrated at the edge of the pattern where heat cannot bereleased.

This phenomenon may become a factor of defective wires composing a thinfilm transistor or of defective operation thereof later.

Thus, the pattern 604 of the active layer is formed so as to have theprofile as shown in FIG. 6B in the arrangement of the presentembodiment.

Such arrangement allows to suppress the pattern of the silicon film frombeing deformed like the one shown in FIG. 7B when laser light isirradiated.

It is preferable to set an angle of the part designated by the referencenumeral 605 from 20° to 50°. It is not preferable to set the angle of605 below 20° because an area occupied by the active layer increases andit becomes difficult to form it. Further, it is not also preferable toset the angle of 600 above 50° because the effect for suppressing theshape as shown in FIG. 7B from being formed is reduced.

The pattern 604 may be realized by utilizing isotropic dry etching inpatterning it and by controlling the conditions of this dry etching.

After obtaining the pattern (which turns out to be the active layerlater) having the shape 603 in FIG. 6B, laser light is irradiated asshown in FIG. 6C. This step allows to diffuse the nickel element whichis locally blocked within the pattern 604 and to promote thecrystallization of the pattern.

After finishing to irradiate laser light, a heat treatment is performedwithin an oxygen atmosphere containing 3% of HCl to form a thermal oxidefilm 606. Here, the thermal oxide film is formed in 100 angstrom thickby performing the heat treatment for 12 hours in the oxygen atmospherecontaining 3% of HCl at 650° C. (FIG. 6D).

The nickel element contained in the pattern 604 is gettered to thethermal oxide film by the action of chlorine. At this time, because theblock of the nickel element has been destroyed through the irradiationof laser light in the previous step, the gettering of the nickel elementis effectively performed.

Further, the gettering is performed also from the side surfaces of thepattern 604 when the arrangement shown in the present embodiment isadopted. This is useful in enhancing the OFF current characteristics andthe reliability of the thin film transistor finally completed. It isbecause the existence of nickel element which promotes crystallizationof silicon and which exists in the side of the active layer relates awide influence over the increase of OFF current and the instability ofthe characteristics.

After forming the thermal oxide film 606 for gettering as shown in FIG.6D, the thermal oxide film 606 is removed. Thus, the state shown in FIG.6E is obtained. It is concerned that the silicon oxide film 602 might beetched in the step of removing the thermal oxide film 606 when thesilicon oxide film is adopted as the underlying layer 602. However, itdoes not matter so much when the thickness of the thermal oxide film 606is as thin as 100 angstrom as shown in the present embodiment.

After obtaining the state shown in FIG. 6E, a new thermal oxide film 607is formed by a heat treatment in an atmosphere of 100% oxygen.

Here, the thermal oxide film 607 is formed in a thickness of 100angstrom by the heat treatment in the oxygen atmosphere at 650° C.

The thermal oxide film 607 is effective in suppressing the surface ofthe pattern 603 from being roughened when the laser light is irradiatedlater. The thermal oxide film also forms a part of a gate insulatingfilm later. Because the thermal oxide film has a very favorableinterfacial characteristic with the crystalline silicon film, it isuseful to utilize it as part of the gate insulating film.

The laser light may be irradiated again after forming the thermal oxidefilm 607. Thus, the crystalline silicon film 604 in which theconcentration of nickel element has been reduced and which has a highcrystallinity may be obtained.

Thereafter, the thin film transistor is fabricated by performing throughthe process shown in FIG. 3 or 4.

Eleventh Embodiment

The present embodiment relates to a case devised in applying a heattreatment at a temperature more than a distortion point of a glasssubstrate. It is preferable to perform the process for gettering themetal element which promotes crystallization of silicon in the presentinvention disclosed in the present specification at a high temperatureas much as possible.

When the Corning 1737 glass substrate (distortion point: 667° C.) isused for instance, the higher gettering effect can be obtained when thetemperature in gettering nickel element by forming the thermal oxidefilm is 700° C. rather than when it is 650° C.

However, if the heating temperature for forming the thermal oxide filmis set at 700° C. using the Corning 1737 glass substrate, the glasssubstrate deforms as a result.

The present embodiment provides means for solving this problem. That is,according to the arrangement shown in the present embodiment, the glasssubstrate is placed on a lapping plate which is formed of quartz whoseflatness is guaranteed and the heat treatment is performed in thisstate.

Thereby, the flatness of the softened glass substrate is maintained bythe flatness of the lapping plate. It is noted that it is also importantto perform cooling in the state in which the glass substrate is placedon the lapping plate.

The adoption of such arrangement allows the heat treatment to beperformed even if it is at the temperature above the distortion point ofthe glass substrate.

1. A method for manufacturing a semiconductor device comprising: forminga semiconductor film comprising amorphous silicon over a substrate;providing the semiconductor film with a metal containing material forpromoting crystallization of the semiconductor film; crystallizing thesemiconductor film by heating; irradiating the crystallizedsemiconductor film with laser light so as to distribute the metal in thecrystallized semiconductor film; removing the distributed metal from thecrystallized semiconductor film by gettering after the irradiation ofthe laser light; forming a semiconductor island having a tapered shapeby patterning the crystallized semiconductor film, the tapered shapehaving an angle within a range of 20° to 50° between a side thereof andan underlying surface; forming a gate insulating film over thesemiconductor island; forming a gate electrode over the semiconductorisland with the gate insulating film therebetween; and forming at leasta source region and a drain region in the semiconductor island.
 2. Amethod for manufacturing a semiconductor device according to claim 1,wherein the patterning is performed by an isotropic dry etching method.3. A method for manufacturing a semiconductor device according to claim1, wherein the metal is selected from the group consisting of Fe, Co,Ni, Ru, Rh, Pd, Os, Ir, Pt, Cu and Au.
 4. A method for manufacturing asemiconductor device according to claim 1, wherein the gettering isperformed by heating the crystallized semiconductor film in a halogencontaining atmosphere.
 5. A method for manufacturing a semiconductordevice according to claim 1, wherein a surface of the crystallizedsemiconductor film is oxidized when the gettering is performed.
 6. Amethod for manufacturing a semiconductor device according to claim 1,wherein the gate insulating film comprises silicon, oxygen, andnitrogen.
 7. A method for manufacturing a semiconductor device accordingto claim 1, wherein the gate insulating film is formed by plasma CVDusing silane and nitrous oxide gases.
 8. A method for manufacturing asemiconductor device according to claim 1, wherein the gate insulatingfilm is formed by plasma CVD using tetraethoxysilane and nitrous oxidegases.
 9. A method for manufacturing a semiconductor device comprising:forming a semiconductor film comprising amorphous silicon over asubstrate; providing the semiconductor film with a metal containingmaterial for promoting crystallization of the semiconductor film;crystallizing the semiconductor film by heating; irradiating thecrystallized semiconductor film with ultraviolet rays or infrared raysso as to distribute the metal in the crystallized semiconductor film;removing the distributed metal from the crystallized semiconductor filmby gettering after the irradiation of the ultraviolet rays or infraredrays; patterning the crystallized semiconductor film by etching to forma semiconductor island; forming a gate insulating film over thesemiconductor island; forming a gate electrode over the gate insulatingfilm; and forming source and drain regions in the semiconductor island.10. A method for manufacturing a semiconductor device according to claim9, wherein the patterning is performed by an isotropic dry etchingmethod.
 11. A method for manufacturing a semiconductor device accordingto claim 9, wherein the metal is selected from the group consisting ofFe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Cu and Au.
 12. A method formanufacturing a semiconductor device according to claim 9, wherein thegettering is performed by heating the crystallized semiconductor film ina halogen containing atmosphere.
 13. A method for manufacturing asemiconductor device according to claim 9, wherein a surface of thecrystallized semiconductor film is oxidized when the gettering isperformed.
 14. A method for manufacturing a semiconductor deviceaccording to claim 9, wherein the gate insulating film comprisessilicon, oxygen, and nitrogen.
 15. A method for manufacturing asemiconductor device according to claim 9, wherein the gate insulatingfilm is formed by plasma CVD using silane and nitrous oxide gases.
 16. Amethod for manufacturing a semiconductor device according to claim 9,wherein the gate insulating film is formed by plasma CVD usingtetraethoxysilane and nitrous oxide gases.
 17. A method formanufacturing a semiconductor device comprising: forming a semiconductorfilm comprising amorphous silicon on an insulating surface; providingthe semiconductor film with a metal containing material for promotingcrystallization of the semiconductor film; crystallizing saidsemiconductor film by heating; irradiating the crystallizedsemiconductor film with laser light so as to distribute the metal in thecrystallized semiconductor film; removing the distributed metal from thecrystallized semiconductor film by gettering after the irradiation ofthe laser light; forming a semiconductor island having a tapered shapeby patterning the crystallized semiconductor film, the tapered shapehaving an angle within a range of 20° to 50° between a side thereof andan underlying surface; forming a first gate insulating film over thesemiconductor island wherein the first gate insulating film comprisessilicon oxide; forming a second gate insulating film over the first gateinsulating film wherein the second gate insulating film comprisessilicon, oxygen, and nitrogen; forming a gate electrode over thesemiconductor island with the first gate insulating film and the secondgate insulating film therebetween; and forming at least a source regionand a drain region in the semiconductor island.
 18. A method formanufacturing a semiconductor device according to claim 17, wherein thepatterning is performed by an isotropic dry etching method.
 19. A methodfor manufacturing a semiconductor device according to claim 17, whereinthe metal is selected from the group consisting of Fe, Co, Ni, Ru, Rh,Pd, Os, Ir, Pt, Cu and Au.
 20. A method for manufacturing asemiconductor device according to claim 17, wherein the gettering isperformed by heating the crystallized semiconductor film in a halogencontaining atmosphere.
 21. A method for manufacturing a semiconductordevice according to claim 17, wherein a surface of the crystallizedsemiconductor film is oxidized when the gettering is performed.
 22. Amethod for manufacturing a semiconductor device according to claim 17,wherein the second gate insulating film is formed by plasma CVD usingsilane and nitrous oxide gases.
 23. A method for manufacturing asemiconductor device according to claim 17, wherein the second gateinsulating film is formed by plasma CVD using tetraethoxysilane andnitrous oxide gases.
 24. A method for manufacturing a semiconductordevice according to claim 1, wherein the semiconductor device isincorporated into a liquid crystal display or an electroluminescencedisplay.
 25. A method for manufacturing a semiconductor device accordingto claim 9, wherein the semiconductor device is incorporated into aliquid crystal display or an electroluminescence display.
 26. A methodfor manufacturing a semiconductor device according to claim 17, whereinthe semiconductor device is incorporated into a liquid crystal displayor an electroluminescence display.